In an electron microscope where the sample being studied is maintained at the same temperature as the objective lens, the sample holder is screw-fastened to the lens, so that vibration or other disturbance does not produce any relative motion between the two, and the image remains in focus. However, sometimes it is desirable to image a specimen in an electron microscope while maintaining the specimen at a low temperature. This requires a heat insulator between the sample holder and the body portion of the stage that is screw-fastened to the objective lens. In this case, relative motion between the two is possible, and the image goes in and out of focus, thus reducing the resolution of the microscope.
In the past, imaging low temperature specimens to high resolution was impossible because of the poor mechanical stability of the cold stages used.
In a typical cold stage of the prior art, the cold stage consists of a brass disc, or stage body, which is screw-fastened onto the stage support at three points. The stage body has a hole in the center which contains an insulating plastic ring into which is bolted the stage "arbor". This arbor is a gold-plated copper annulus with its center hole cut at a taper to accommodate the specimen cartridge. The arbor is cooled by a length of copper braid, which is soldered to the arbor and bolted to a copper cold finger that extends into the column from a liquid nitrogen dewar employed as a cooling source. This stage requires approximately 11/2 hours to cool down and produces a steady-state temperature of about -120.degree. C. at the top of the specimen cartridge. Its chief stability problem appears to be the generation of a drumhead-type motion of the plastic insulating ring which prevents the arbor from being ridigly coupled to the stage body. Vibrational input causing lack of vertical synchrony between the arbor and the objective lens polepiece apparently is derived from one or more of three sources: (1) ambient floor vibrations transmitted to the stage body, (2) vibrations of the cold finger due to amplification of ambient floor vibrations by the long lever arm of the dewar to the column, and (3) vibrations of the cold finger due to boiling of the liquid nitrogen.
In general, the results obtainable with the above-described prior art cold stage are not satisfactory for imaging specimens at low temperature, for example, for imaging purple membranes, requiring a liquid nitrogen-cooled stage stable to 3.5 Angstroms.
A preliminary search of the patented prior art revealed the following prior U.S. Pat. Nos. of interest:
Leisegang 2,858,444 PA1 Herrmann et al, 3,151,241 PA1 Matthews 3,230,773 PA1 Kolenko, 3,297,491